Responses to the Tohoku–Pacific Ocean Earthquake andTsunami at the Onagawa Nuclear Power Station andTokai No.2 Power Station (report)

August 2013

Japan Nuclear Safety InstituteTable of Contents
1

Preface................................................................................................................................................................ 1

2

Overview of the Tohoku–Pacific Ocean Earthquake and Tsunami .................................................................... 2
2.1

3

4

5

6

7

8

9

Overview of the earthquake and tsunami..................................................................................................... 2

3 Overview of the Onagawa Nuclear Power Station .......................................................................................... 6
3.1

Overall layout .............................................................................................................................................. 6

3.2

System configuration ................................................................................................................................... 7

3.3

Power source system.................................................................................................................................... 9

Damage caused by the earthquake and tsunami at the Onagawa Nuclear Power Station .................................11
4.1

Observation results at the Onagawa Nuclear Power Station.......................................................................11

4.2

Damage and impact from the earthquake .................................................................................................. 14

4.3

Damage and impact from tsunami ............................................................................................................. 16

Responses to the earthquake disaster at the Onagawa Nuclear Power Station................................................. 19
5.1

Overview of responses right after the earthquake and tsunami for recovery and cold shutdown.............. 19

5.2

Situation of the earthquake disaster responses........................................................................................... 19
5.2.1

Immediately after the earthquake occurred ....................................................................................... 19

5.2.2

Immediately after tsunami arrived .................................................................................................... 21

5.2.3

Measures for recovery after tsunami arrived..................................................................................... 22

Lessons learned from the earthquake disaster responses at the Onagawa Nuclear Power Station ................... 29
6.1

Organization, management, communication ............................................................................................. 29

6.2

Preparedness (system, manual, training) ................................................................................................... 29

6.3

Initial responses in the event of the earthquake ......................................................................................... 30

6.4

Additional measures .................................................................................................................................. 30

Overview of the Tokai No.2 Power Station...................................................................................................... 31
7.1

Overall layout ............................................................................................................................................ 31

7.2

System configuration ................................................................................................................................. 32

7.3

Power source system.................................................................................................................................. 34

Damage caused by the earthquake and tsunami at the Tokai Dai-ni Power Station ......................................... 35
8.1

Observation results at the Tokai No.2 Power Station ................................................................................ 35

8.2

Damage and impact from the earthquake .................................................................................................. 36

8.3

Damage caused by tsunami ....................................................................................................................... 36

Responses to the earthquake disaster at the Tokai No.2 Power Station ........................................................... 38
9.1

Overview of responses right after the earthquake and tsunami for recovery and cold shutdown.............. 38

9.2

Situation of the earthquake disaster responses........................................................................................... 38
9.2.1

Immediately after the earthquake occurred ....................................................................................... 38

9.2.2

Immediately after the tsunami arrived .............................................................................................. 40

9.2.3

Plant responses after the tsunami arrived at the plant ....................................................................... 40

10 Lessons learned from earthquake disaster responses at the Tokai No.2 Nuclear Power Station ...................... 44
10.1 Organization, management, communication ............................................................................................. 44i10.2 Preparedness (system, manual, training) ................................................................................................... 44
10.3 Initial responses in the event of an earthquake .......................................................................................... 45
10.4 Additional measures .................................................................................................................................. 45
11 Conclusion ....................................................................................................................................................... 46ii

1

Preface
With the cooperation from electric power companies and plant manufacturers, the Japan Nuclear Technology
Institute, the predecessor of the Japan Nuclear Safety Institute, summarized lessons learned concerning the
Tohoku–Pacific Ocean Earthquake and Tsunami-induced accident at the Fukushima Dai-ichi Nuclear Power
Station run by the Tokyo Electric Power Company, Inc. (TEPCO). It then prepared and published a report in
October 2011. This report summarizes 80 items mainly consisting of hardware measures to prevent accidents
and mitigate their consequences, and strongly recommends that an additional 23 items of measures are
implemented since they are considered to be important in terms of defense-in-depth. These measures havebeen implemented as necessary depending on the systems and conditions of each company and almost allitems have already been implemented or their implementation is under consideration.
Moreover, the Fukushima Dai-ni Nuclear Power Station run by TEPCO, Onagawa Nuclear Power Station,
which in turn is run by the Tohoku-Electric Power Co., Inc., and Tokai No.2 Power Station, which is run by
The Japan Atomic Power Company could have prevented the nuclear disaster and shut down the plant safely
in the face of impacts caused by the earthquake and tsunami. With support from TEPCO, the Japan Nuclear
Safety Institute performed an analysis on the earthquake disaster responses at the Fukushima Dai-ni Nuclear
Power Station, which sustained relatively extensive damage among these stations, in terms of human factors
and organizations. The Institute extracted lessons learned mainly from a software aspect, and developed andpublished the report in December 2012.
These measures are not like those that will be completed once implementation has been done, but more likemeasures that should be improved continuously through exercises and the like.
The above two reports are available from the website of the Japan Nuclear Safety Institute; please have a readthrough them since they have created momentum to prepare this report.
With the objective of preserving the records of the earthquake disaster responses at the remaining two nuclear
power stations, Onagawa Nuclear Power Station and Tokai No.2 Power Station, a review team wasestablished within the Japan Nuclear Safety Institute. The team consolidated the earthquake disaster
responses at each power station and summarized the lessons learned, in cooperation with Tohoku-ElectricPower Co., Inc. and The Japan Atomic Power Company.
At the Onagawa Nuclear Power Station, a site elevation was secured that far exceeded the anticipated tsunami
height, as a conservative approach when determining the site elevation during construction. Therefore, themain body of the plant received little damage from the tsunami, enabling it to have the accident situation
settle down. In addition, construction works to strengthen watertightness as measures against tsunami were
underway at the Tokai No.2 when the earthquake occurred. Thus, a loss of all seawater pump functions could
be avoided and this led to the accident situation settling down though there was a little damage in the area
where construction works were unfinished. It is advisable to consider the way the organization is managed toclarify why these power stations took such measures and TEPCO did not.
At this time, this report summarizes the responses at both power stations during the accident and extracts thelessons learned, and the Institute expects this report to be utilized as a reference when each company
addresses safety enhancement.

1

2

Overview of the Tohoku–Pacific Ocean Earthquake and Tsunami
2.1 Overview of the earthquake and tsunami
The Tohoku–Pacific Ocean Earthquake occurred at 14:46 on March 11, 2011 and was the biggest
earthquake on record in Japan; a main shock and maximum seismic intensity of 7 was observed in
Kurihara City, Miyagi Prefecture in this earthquake.
In addition, a very large tsunami was observed on the Pacific coasts of Hokkaido, Tohoku and Kanto
districts.
The seismic source of this earthquake was off the coast of Sanriku at a latitude of 38.1 degrees north andlongitude 142.9 degrees east in the east-southeast 130 km from Oshika Peninsula and the focal depth was
24 km. The focal area extended from off the coasts of the Iwate to Ibaraki Prefectures with a length about
500 km and width of about 200 km. Moreover, the reported maximum slippage exceeded 50 m.
In this earthquake, a large amount of slippage was observed in the areas close to the sea trench in the south
area off the coast of Sanriku as well as close to the sea trench through the north area off the coast of
Sanriku to off the coast of Boso. This was a giant earthquake with a magnitude of 9.0 (the fourth largest on
the world record) and it occurred simultaneously in multiple areas including the central area off the coastof Sanriku, and off the coasts of Miyagi, Fukushima and Ibaraki Prefectures as the focal area. Though The
Headquarters for Earthquake Research Promotion, the national survey & research facilities, had evaluatedearthquake motion and tsunami in the individual areas with prior occurrences, an earthquake that occurs
simultaneously in all these areas was not anticipated. The Special Committee at the Central Disaster
Prevention Council also reported that a giant earthquake with a magnitude of 9.0, which could not be
anticipated based on earthquake records for the past several centuries in Japan, did occur simultaneously inmultiple areas as an earthquake with an extensive focal area.
The tsunami that was generated subsequent to this earthquake and induced the large-scale disaster on the
Pacific coast of the Tohoku District had a tsunami magnitude (indicates a scale of tsunami) of 9.1 and was
the fourth largest on the world record and the largest in Japan.

2Isoseismal map off the coast of Sanriku at around 14:46 on March 11, 2011

LegendIntensity 7
Intensity 6
upper
Intensity 6
lower
Intensity 5
upper
Intensity 5
lower
Intensity 4
Intensity 3
Intensity 2

Epicenter

Intensity 1

Source: Japan Meteorological Agency (Earthquake off the cost of Sanriku at around 14:46 on March 11, 2011)

3

Function of time at theseismic source

Seconds
Hypocenter of the main shock
Epicenter over M7 earthquake after March 9
Epicenter over M5 earthquake during a day
after the main shock
Center point of each small fault
Seismic station for analysis

Slippage (m)At 4-meter intervals for contour(Contour: Profile, profile line or level line)
Source: Japan Meteorological Agency (Monthly Report on Earthquakes and Volcanoes issued in March 2011)
It was assumed that a tsunami occurred because the ocean bed almost directly above the seismic source
heaved by approx. 3 m. The maximum runup height was nearly 35 min the north area of Miyako City. In
addition, the inundation height in the north area of Miyako City exceeded 25 m. The inundation areas were
58 km2 in Iwate, 327 km2 in Miyagi, 112 km2 in Fukushima and 23 km2 in Ibaraki.

4[Legend]

Trace of tsunami
AomoriPrefecture
Hachinohe

Inundation height off
the coast of Tohoku
Runup height off thecoast of Tohoku

Miyako
Iwate
Prefecture
Rikuzentakata

Sendai

MiyagiPrefecture

Fukushima
Prefecture

IbarakiPrefecture(Source)• Inundation and runup heights of the
Tohoku–Pacific Ocean Earthquake
2011: Quick estimations by the “Joint
Survey Group on the Tohoku–Pacific
Ocean Earthquake and Tsunami”(May 9, 2011). Note: Data used are with
Reliability “A” (high reliability, clear
trace with the smallest measurement
error) within 200 m from the coast.

Inundation and runup heights
Inundation
heightSea dike

RunupheightTidal level when tsunami hit
Average sea surface in Tokyo
Bay (T.P.)

Inundation height: From the tidal level when tsunami hit to the height of tsunami trace
Runup height: From the tidal level when tsunami hit to the height of tsunami running up

Source: Expert Examination Committee on Earthquake and Tsunami Measures with Lessons
Learned from the Tohoku–Pacific Ocean Earthquake
Extracted from the 1st meeting material
The damage caused by the earthquake and tsunami was extensive (15,883 deaths, 2,667 missing, 126,467
fully destroyed houses, 272,244 partially destroyed houses as of July 10, 2013) (Source: Materials issued by
the National Police Agency on July 10, 2013)

5

3

3

Overview of the Onagawa Nuclear Power Station

3.1 Overall layout
Onagawa Nuclear Power Station is located in almost the central eastern part of Oshika Peninsula in Miyagi
Prefecture and its northeast side faces the Pacific Ocean. The site is surrounded by mountains on threesides and consists of a mountainous district and narrow flatland. It has an almost semicircular shape where
the diameter is the coastline side, and a site area of approx. 1,730,000 m2.
Currently, it has three boiling water reactors and Units 1 & 2 are located in the southeast side of the sitefrom the mountain side toward the seaside and Unit 3 in the southwest side. The generator outputs are
524,000 kW at Unit 1 and 825,000 kW at Units 2 & 3 and the total capacity of nuclear power generation is2,174,000 kW.
When the Tohoku–Pacific Ocean Earthquake occurred, Units 1 & 3 were in constant-rated thermal power
output operation and Unit 2 was outage and reactor startup had been initiated from 14:00.

North seawall

East seawall
Water discharge canal(Units 2 & 3)
Water discharge canal(Unit 1)IntakeNuclear Technology
Training Center

Unit 2

Environmental
radioactivity
measurement center

Unit 3Ishinomaki
City

Unit 1
Onagawa gate
Office building
Access road

Onagawa Town

Maintenance andrepair center

Figure 3.1 Overall layout of the power station (when the earthquake occurred)

Unit

Start ofoperation

ReactorType
Containment

Output(10,000 kW)

1June 1984

BWR-4

Mark I

52.4

2

July 1995
BWR-5

Mark I
Advanced

3

January 2002

82.5
82.5

6

Status in the event of the earthquake
Constant-rated thermal power output
operation
Undergoing reactor startup (periodic
inspection)
Constant-rated thermal power output
operation

3.2 System configuration
The system configurations of each unit at Onagawa are shown in Figures 3.2-1 and 3.2-2.
The roles of each system are as follows:• Reactor core isolation cooling system (RCIC)
If the main condenser becomes unavailable during normal operation due to the closed main steam
isolation valve or such like due to any cause, the system rotates the turbine-driven pumps by the reactor
steam and injects the water into the reactor from the condensate storage tank to remove fuel decay heatand depressurize. In addition, the system is used as an emergency injection pump to maintain thereactor water level in the event of a feedwater system failure or such like.
• Residual heat removal system (RHR)
After reactor shutdown, the system cools the coolant by using pumps and heat exchangers (removal offuel decay heat), maintains the core water by injecting cooling water in case of an emergency (one of
the emergency core cooling systems) and can bring the reactor to a cold shutdown. It has five operation
modes: the reactor shutdown cooling mode, low-pressure injection mode (emergency core coolingsystem), containment spray mode, suppression chamber cooling mode and fuel pool cooling mode.
• Emergency core cooling system (ECCS)
It consists of a low-pressure core spray system (LPCS), low-pressure core injection system (LPCI),
high-pressure core injection system (HPCI), high-pressure core spray system (HPCS) and automatic
depressurization system (ADS). It removes fuel decay heat and residual heat from the core, prevents
damage to the fuel cladding caused by overheated fuels and controls the subsequent zirconium-water
reaction to a negligible level if the piping, consisting of the reactor coolant pressure boundary such as
the reactor recirculation system piping ruptures and results in a loss of coolant accident (LOCA).

7

Reactor
containment

Main turbine

High-pressure core
injection system (HPCI)

Feedwater pump
(three motoroperated pumps)

Condensate
pumpCondenser(three pumps)

Reactor coreisolation coolingsystem (RCIC)

Circulating water
pump (two pumps)
Reactorpressure vessel

To the suppression
chamberSea

To the suppression
chamber

Condensate
storage tank

Borated water storage tank

Suppression
chamber

Residual heatremoval system
heat exchanger

Control rod drive hydraulic system (CRD) (two systems)

Residual heatremoval system
heat exchanger

Make-up water condensate system(MUWC) (two systems)

Residual heat removal
seawater system (RHRS)

Standby liquid control system (SLC)

To the water
discharge canal(sea)

Residual heat removal system (RHR)
To the water discharge canal (sea)Core spray system (CS)Residual heatremoval seawater
system (RHRS)

Sea

Sea

Figure 3.2-1 System configuration at Unit 1 of the Onagawa Nuclear Power Station

Reactor core isolation
cooling system (RCIC)

Reactorcontainment
Feedwater pump

Main turbine

Condenser
High-pressure core spraysystem (HPCS)

Two motor-driven
turbines

To the suppression
chamber

Two motor-driven
pumps

Condensate pumpThree highpressure pumps

Threelow-pressure
pumps

Circulating water pump
(two pumps)

Reactorpressure vessel

Sea

Condensate water
storage tank/condensate
water storage reservoir

Borated water storage tank

Suppression
chamber

Low-pressure core spraysystemControl rod drive hydraulic system(CRD) (two systems)

Make-up water condensate system (MUWC) (three systems)

Reactor building closed-cooling-water
systemReactor building closed coolingseawater system

Standby liquid control system (SLC)

Residual heat removal system (RHR) C
Residual heat removal system
heat exchanger

Residual heat removal system (RHR) B

Reactor building closed-cooling-water
systemReactor building closed coolingseawater system

Residual heat removal system
heat exchanger

Residual heat removal system (RHR) A

Sea

Sea

Figure 3.2-2 System configuration at Units 2 & 3 of the Onagawa Nuclear Power Station

8

3.3 Power source system
The electricity generated at each unit is sent to the power source system via 275 kV transmission lines withfour circuits. This 275 kV transmission line can be used to transmit all the electricity generated at the
Onagawa Nuclear Power Station even if one circuit is unavailable. In addition, it can conduct full-power
operation at the station in the event of an accident in one circuit.
Should all four circuits of the 275 kV transmission line lose power, the electricity for the system to safely
shut down the reactor will be supplied from the emergency diesel generator, high-pressure core spraysystem (HPCS), diesel generator or one circuit of the 66 kV transmission line.
One circuit of the 66 kV transmission line is shared among Units 1 – 3 and it receives power via theemergency transformer.

910

Unit 2

Unit 2
HTr (A)

Unit 2 MTr

Unit 2
HTr (B)

Unit 1 STr

Unit 1
HTr (A)

Oshika main line 2

Unit 1
HTr (B)

Unit 1 MTr

Standbytransformer

Tsukahamabranch line
Unit 3STr (A)

Matsushima main line 1

Figure 3.3-1 Power source system diagram, emergency power source system skeleton diagram

Unit 2 STr

Oshika main line 1

Unit 3STr (B)

Unit 3HTr (B)

Unit 3 MTr

Matsushima main line 2

Unit 3

4

Damage caused by the earthquake and tsunami at the Onagawa Nuclear Power Station
4.1 Observation results at the Onagawa Nuclear Power Station
The maximum acceleration rates observed at each floor of the reactor buildings at Onagawa Units 1, 2 and3 were almost equivalent, though some results exceeded the maximum response acceleration value for thebasic design earthquake ground motion Ss.
As a result of stripping analysis conducted on earthquake records at an elevation equivalent to the free rocksurface (O.P -8.6m) observed and acquired by the seismograph on the site ground during the main shock, it
was confirmed that the quake became bigger in a short cycle and the observation record exceeded the basic
design earthquake ground motion Ss in some periodic bands, the same as before performing the stripping
analysis.
Moreover, seismic response analysis was performed based on the earthquake observation record toevaluate the deformation of the seismic wall of the reactor buildings at Onagawa Units 1–3 as well as theshear force that affected the seismic wall of each floor. Consequently, it was confirmed that the reactor
building function was maintained.
The entire site sank by approx. 1 m due to the earthquake.
Table 4.1-1 Comparison between observation records and maximum response acceleration value for the basic design
earthquake ground motion Ss*1 (Unit: Gal)
Observation record (maximum acceleration

Maximum response acceleration value forvalues)

Ss

Observation point
North-south

East-west

VerticalNorth-south

East-west

Vertical

Roof

2000

*2

1636

1389

2202

2200

1388

Refueling floor (5F)

1303

998

1183

1281

1443

1061

1F

573

574

510

660

717

527

Above base mat

540

587

439

532

529

451

Roof

1755

1617

1093

3023

2634

1091

Refueling floor (3F)

1270

830

743

1220

1110

968

1F

605

569

330

724

658

768

Above base mat

607

461

389

594

572

490

Roof

1868

1578

1004

2258

2342

1064

Refueling floor (3F)

956

917

888

1201

1200

938

1F

657

692

547

792

872

777

Above base mat

573

458

321

512

497

476

Unit 1

Unit 2

Unit 3

Source: Tohoku-Electric Power Co., Inc. (Report overview of the earthquake/tsunami survey result dated April 7, 2011)

Table 4.1-2 Earthquake observation records and maximum acceleration values based on the stripping analysis (Unit: Gal)
North-south

East-west

VerticalEarthquake observation record

467

421

269

Stripping analysis result

517

636

312

Basic design earthquake
580

387

ground motion Ss
Source: Tohoku-Electric Power Co., Inc. (Overview of the stripping analysis result dated December 9, 2011)11

Basic design earthquake ground motion Ss-D (vertical direction)

Basic design earthquake ground motion Ss-D (horizontal direction)

Basic design earthquake ground motion Ss-B (vertical direction)

Basic design earthquake ground motion Ss-B (horizontal direction)

Earthquake observation record (vertical direction)

Earthquake observation record (north-south direction)

Acceleration(cm/s)

Acceleration(cm/s)

Earthquake observation record (east-west direction)

Cycle (second)

Cycle (second)

3.11 earthquake horizontal direction

3.11 earthquake vertical direction
Stripping analysis result (vertical direction)

Stripping analysis result (south-north direction)

Acceleration(cm/s)

Acceleration(cm/s)

Stripping analysis result (east-west direction)

Cycle (second)

Cycle (second)

3.11 earthquake horizontal direction

3.11 earthquake vertical direction

Figure 4.1-1 Response spectra of the earthquake observation results (above) and stripping analysis result (bottom)
Source: Tohoku-Electric Power Co., Inc. (Overview of the stripping analysis result dated December 9, 2011)12

(1) Maximum acceleration
2nd basement of the Unit 1 reactor building: 567.5 gal(2) Distance from the Onagawa Power Station
Distance to the epicenter: 123 km; distance to the seismic source: 125 km(3) Tsunami data(i)

Inundation height
O.P. + approx. 13 m*3(ii)

Inundation area
Not exceeding the site elevation (O.P. + approx. 13.8 m*3). Tsunami did not reach the majorbuildings though there was a trace of seawater inundation in part of the seaside at the site.

(4) Time when the maximum tsunami height was reached at the site
At 15:29 on March 11, 2013
*1:If there are multiple observation points in horizontal and vertical directions, the maximum value
is indicated.
*2:Reference value because the observed value exceeded the maximum setpoints of the pertinent
seismograph (2000 gal)
*3:The average sea surface in Tokyo Bay (T.P.) = O.P. + 0.74 m and crustal deformation (approx. -1
m: Quick estimation) around the Onagawa Nuclear Power Station, which the Geospatial
Information Authority of Japan announced after the earthquake, was taken into consideration.

13

4.2 Damage and impact from the earthquake
Five circuits (Oshika main line 1, 2 (275 kV), Matsushima main line 1, 2 (275 kV), and Tsukahama branchline (66 kV)) are connected to Onagawa as off-site power sources. Shortly after the earthquake, only
Matsushima main line 2 with one circuit was available due to operation of the system protection circuits inassociation with the transmission line accident that occurred in the area managed by Tohoku-ElectricPower Co., Inc. Then Oshika main line 1 was recovered at 20:12 on March 12, Oshika main line 2 at 20:15
on the same day, Matsushima main line 1 at 10:47 on March 17 and Tsukahama branch line at 15:41 on
March 26.

Recovered at 20:12 on Mar 12
Shutdown for inspection at
19:32 on Mar 18

Recovered at 20:15 on
Mar 12Power
receivable

Matsushima
main line 1

Oshikamain line 2

Oshika main line 1

Onagawa
substation

Miyagi central switchyard

Ishinomaki substation

Matsushima
main line 2

Recovered at 15:41 on
Mar 26

Recovered at 10:47 on
Mar 17

Unit 2
STr

Unit 2
MTr

Unit 1
STr

Generator

Unit 2high-voltage
power panel

Unit 2high-voltage
power panel

Unit 2high-voltage
power panel

Unit 1
MTr

Unit 3
STr

Unit 3
STr

Generator

Unit 1high-voltage
power panel

Unit 1high-voltage
power panel

Tsukahamabranch line

Unit 3
MTr

Standbytransformer

Generator

Unit 3high-voltage
power panel

Unit 3high-voltage
power panel

Unit 3high-voltage
power panelStandbyUnit 2systemUnit 2systemUnit 2systemStartup

Unit 1systemUnit 1systemUnit 3systemUnit 3systemUnit 3systemLegend: Supply
MTr: Main transformer
STr: Startup transformer
DG: Emergency diesel generator
DG (H): High-pressure core spray system emergency diesel generator
Power source system status of Units 1-3 at the Onagawa Nuclear Power Station (shortly after the earthquake on March 11)

Figure 4.2.1 Off-site power source schematic diagram
Source: Tohoku-Electric Power Co., Inc. (situation at Onagawa concerning the earthquake and subsequent tsunami)

Damage situation
Among the damaged systems, the following describes the situation of damage to the systems related tothe power source and reactor cooling that clearly indicated characteristics of system damage caused bythis earthquake.
a. High-voltage power distribution panel fire (Unit 1)
At 15:30 on March 11, smoke was found in the turbine building, 1st basement at Unit 1 and firefighting
activities were performed. It was confirmed that the fire was extinguished at 22:55 on March 11 and it
was identified that smoke was being generated by the high-voltage power distribution panel.
The cause was presumed to be a suspended breaker*1 inside the high-voltage power distribution panelthat had swung widely due to the seismic vibration and damaged the disconnecting parts of thepertinent breaker, resulting in the short circuit and such like by touching the surrounding structuresinside the high-voltage power distribution panel. Subsequently, the cable insulation coating inside thehigh-voltage power distribution panel melted because of the heat induced by arc discharge and smoke
was emitted.
14

b. Loss of the emergency diesel generator (A) functions (Unit 1)
In the periodic testing on the emergency diesel generator (A) at the Onagawa Nuclear Power Station
Unit 1 performed on April 1, the synchronoscope*2 to connect to the on-site power source system was
not operating; thus, it was not possible to connect to the on-site power source system.
Because of that, the inspection was performed for the synchronoscope circuit. During inspection, an
event occurred where the breaker of the emergency diesel generator (A) was automatically loaded in astate where the emergency diesel generator (A) was not activated. Therefore, the inspection for the
emergency diesel generator (A) was initiated from April 5.
As a result of the inspection, damage to the circuit used for voltage adjustment and such like of theemergency diesel generator was confirmed.
The presumed cause was an earth fault that occurred in the cable connecting the synchronoscope
circuits affected by fire on the high-voltage power distribution panel, and this caused the fuse to blow
due to the earth fault current that flowed when the synchronoscope switch was turned; thus thesynchronoscope was not operated.
In addition, the cable insulation coat connecting the synchronoscope circuits melted, affected by thefire of the pertinent high-voltage power distribution panel and another piece of insulation coating
touched the melted cable, leading to the situation where a current flowed. Therefore, it was assumed
that the breaker for the emergency diesel generator (A) was automatically loaded while detachingcircuits from the high-voltage power distribution panel in association with the synchronoscope
inspection, and this caused an overvoltage on the circuits used to adjust the voltage from the on-site
power source system to the emergency diesel generator (A), resulting in damage.
After it had been confirmed that the fuse of the synchronoscope had blown and that the circuits usedfor voltage adjustment had been replaced, an operation check was performed on May 18 and it was
confirmed that the emergency diesel generator (A) had become operable.
c. Spent fuel pit cooling & clean up system stoppage (all Units)
Though the spent fuel pit cooling & clean up system stopped at 14:47 on March 11 due to theearthquake, it was restarted after checking that there was no damage to the system. During the
stoppage, there was no significant increase in the spent fuel pit temperature.
The cause might be that the pump suction pressure of the spent fuel pit cooling & clean up systemdecreased because the level switch for “skimmer surge tank level low-low” was operated due to theearthquake or there was a temporary decrease of the spent fuel pit level due to the seismic shaking.
d. Startup transformer stoppage (Unit 1)
As for the status of the power source, power was distributed to the site from Matsushima main line 2 as
the off-site power source via the startup transformer. Since the startup transformer stopped at 14:55 on
March 11, however, the on-site power source was unavailable and then the emergency bus received
power from the emergency diesel generators (A) and (B) as designed.
The cause was that earth fault and short circuit occurred inside the normal metal-clad switch gear 6-1A
among the high-voltage power distribution panels (fire occurred subsequently) and the overcurrent
relay for the startup transformer was activated. Then, the startup transformer was recovered at 02:05 on
March 12 after checking that there was no abnormality in the startup transformer by carrying out avisual inspection and measuring insulation resistance. After the startup transformer was recovered,
power for normal buses other than the normal metal-clad switch gear 6-1A was recovered in series.
*1: Component that automatically detaches the electric circuit when connecting the electric circuits orovercurrent flows.
*2: Component that checks whether the electric nature (voltage, frequency) is equivalent to mitigate
shock when manually connecting the diesel generator to the on-site power source system forperiodic testing or such like. During loss of off-site power supply, the emergency diesel generator
15

will be connected automatically without using the synchronoscope because there is no electricity
for the on-site power source system.
4.3 Damage and impact from tsunami
The tsunami height observed by the tide indicator after the earthquake was O.P. + approx. 13 m* at the
maximum and it was confirmed that this height did not exceed the site elevation of Onagawa (O.P. +approx. 13.8 m*). Though a trace of inundation was partly seen in the seaside of the site due to runup,
the tsunami did not reach the major buildings.
There was an awareness that ensuring measures against a tsunami was the most important issue since
the early planning stages of the Onagawa Nuclear Power Station Unit 1, and this issue had been
discussed at review meetings involving external experts. Then it was determined that the site elevation
should be 14.8 m by incorporating the review result, “The site elevation serves as a measure against atsunami. It should be around 15 m above sea level.” Consequently, this contributed to minimizing thedamage caused by the tsunami.
Seawater pumps themselves are stored in pits, which were made by digging down to a lower elevation(O.P. + 2.5 m) than the site elevation (O.P. + approx. 13.8 m*) so that pumps can be stored in light ofthe site elevation as measures against a tsunami. In addition, the pump base and such like arewaterproofed so that seawater cannot enter via the cooling water intake canal in the event of a tsunami.
The cooling water discharge canal has shafts.
*

A value that considers the crustal deformation (approx. -1 m) around the Onagawa Nuclear Power
Station, which the Geospatial Information Authority of Japan announced after the earthquake.
LegendTrace height, runup height
Point where trace of tsunami can beseen
Site where major buildings are located

Runup height: O.P. + 13.8 m

Figure 4.3-1 Trace survey result
Source: Tohoku-Electric Power Co., Inc. (tsunami survey result July 8, 2011)16

Damage situation
Among the damaged systems, the following describes the situation of damage to the systems related tothe power source and reactor cooling that clearly indicated characteristics of the system damage causedby this tsunami.
a. Heavy oil storage tank collapse (Unit 1)
It was confirmed that the heavy oil storage tank, placed outdoors for the auxiliary boiler*1 of the
Onagawa Nuclear Power Station Unit 1, collapsed and heavy oil was leaking.
The cause was determined to be that the pertinent tank was placed at EL 2.5 m from the mean sea
level, which is lower than the site elevation (approx. 13.8 m*) at which major systems in the stationsite are located, thus it collapsed because of the tsunami. Since there was no damage to the oil wall
surrounding the tank, it was assumed that tank floated in the tsunami and collapsed over the oil wall.
The leaked heavy oil (approx. 600 kl) was collected by using an adsorption mat as much as possible
to reduce diffusion and then a double-walled oil fence was installed inside the bay.
* A value that considers the crustal deformation (approx. -1 m) around the Onagawa Nuclear
Power Station, which the Geospatial Information Authority of Japan announced after theearthquake.

Figure 4.3-2 Collapsed heavy oil tank
Source: Tohoku-Electric Power Co., Inc. (Power Station letter dated June 2011)b. Loss of functions at the reactor building closed-cooling-water system (B), reactor building closed
cooling seawater system (B) and high-pressure core spray component cooling water system (Unit 2)
As seawater inundation was seen at the closed- cooling-water system heat exchanger room located at
the reactor building 3rd basement of Unit 2 in the non-controlled area, the inundated reactor building
closed-cooling-water system pump*2 (B) motors and high-pressure core spray component coolingwater system pump*3 motors in the closed-cooling-water system heat exchanger room were found to17

be inoperable after inspection.
When a signal indicating a loss of the generator excitation was sent, induced by the seismic shaking,the emergency diesel generator and high-pressure core spray system diesel generator wereautomatically activated. Then the emergency diesel generator (B) and high-pressure core spraysystem diesel generator automatically stopped, affected by water inundation. However, the on-site
power source was secured because the off-site power source supplied power. In addition, theconditions meant that the workers could have secured the necessary power by starting up the
emergency diesel generator (A) even if the off-site power source had been lost.
The cause of the seawater inundation might be that the upper lid of the water level gauge box locatedin the seawater pump room was lifted due to an increase in the water level caused by the tsunami.
Then the seawater invaded through this gap, flowed into the basement trench via a cable tray andpipe penetration, and then entered the reactor building closed-cooling-water system heat exchanger
room and other locations via pipe penetrations.
At a later date, the pertinent water level gauge was removed to make the opening waterproof. The
plan was to relocate the pertinent water level gauge to a different area with a dust extractor.
Though the same water level gauges were installed at Units 1 and 3, there was no damage to thereactor building closed-cooling-water system because the installation area was different (area withdust extractor).
*1: The system that supplies steam to heat the gland steam generator which supplies steam to be used
for heating and such like inside the power station buildings as well as sealing steam for theturbine axis sealing part during plant startup (steam to be supplied to the axis sealing part toprevent turbine steam from leaking from the rotor).
*2: The pump that circulates water to cool the emergency diesel generator and residual heat removal
system and such like.
*3: The pump that circulates water to cool the high-pressure core spray system diesel generator andthe high-pressure core spray pump motors and such like that feed water to the reactor when thereactor water level drops abnormally.
Site elevation(O.P. + 13.8 m*)

Reactor building annex(non-controlled area)

Reactor building(controlled area)

Water level gauge penetration for thecirculating water pump automatic stop
Seawater pumpObservation record of the tide
indicator O.P + approx. 13 m*
Tsunami height indicatedin the licensing forestablishment

Seawaterinvasion path
Heat exchanger

Mean sea level 0 m
Water intake
canalComponent cooling
Piping, cable,
water system pump
culvert and such like
(O.P. is the mean sea level at Onagawa and -0.74 m from the mean sea level in Tokyo Bay)*A value that considers the crustal deformation (approx. -1 m) around the Onagawa Nuclear Power Station, which
the Geospatial Information Authority of Japan announced after the earthquake.
Figure 4.3-3 Inundation path to the reactor building closed-cooling-water system heat exchanger (B) room andsuch like at Onagawa Unit 2 (schematic view)Source: Tohoku-Electric Power Co., Inc. (compliance report dated May 30, 2011)18

5

Responses to the earthquake disaster at the Onagawa Nuclear Power Station
5.1 Overview of responses right after the earthquake and tsunami for recovery and cold shutdown
As Units 1 and 3 were in constant-rated thermal power output operation and Unit 2 was undergoing areactor startup at the Onagawa Nuclear Power Station, “a seismic acceleration high” signal was sent,
induced by the Tohoku–Pacific Ocean Earthquake and reactors at all units were automatically shut down.
Since Matsushima main line 2 remained functional out of the five circuits as the off-site power source,
Units 2 and 3 received power via the startup transformer and the on-site power was secured.
At Unit 1, the emergency diesel generator was activated because the startup transformer stopped after theearth fault of the normal buses caused by the earthquake. In addition, the reactor core isolation coolingsystem was activated for cooling the reactor by utilizing power from the DC power source. Pressure wascontrolled through the main steam relief valves. After depressurization, the control rod drive hydraulicsystem fed the water to the reactor. The residual heat removal system was used to cool the suppression
chamber and reactor, resulting in a cold shutdown at 00:58 on March 12.
At Unit 2, the reactor was subcritical with a reactor water temperature of less than 100ºC just before theearthquake; therefore, the reactor entered a cold shutdown condition by putting the reactor mode switch in
the “shutdown” position at 14:49 on March 11. Seawater flowed in from the cooling water canal of theseawater pump room, intruded the reactor building via the underground trench and caused loss of functionsin the reactor building closed-cooling-water system (B) and high-pressure core spray component coolingwater system. However, the reactor cooling function was secured by the residual heat removal system (A)because the reactor building closed-cooling-water system (A) was intact. Moreover, it was assumed thatthe circulating water pump automatically stopped because the water level detector for seawater pumps wasdamaged by anaseism.
At Unit 3, the circulating water pump automatically stopped as the water level detector for seawater pumps
was damaged by anaseism and the signal was sent indicating a very low water level in the seawater pump
room. Furthermore, the reactor core isolation cooling system was manually activated for reactor coolingafter stopping the reactor feedwater pumps because the turbine building closed cooling seawater systemlost its functions. Pressure was controlled through the main steam relief valves. After depressurizing thereactor, MUWC fed water to the reactor. The residual heat removal system was used to cool the
suppression chamber and reactor, resulting in a cold shutdown at 01:17 on March 12.
Cooling systems for the spent fuel pool also stopped automatically, affected by the seismic shaking;however, they were resumed after checking that there was no abnormality in the systems, and no
significant increase in temperature was detected.
As for the off-site power source, Oshika main lines 1 and 2 were recovered at 20:12 and 20:15 the next day(March 12) respectively. In addition, the startup transformer for Unit 1 was recovered at 02:05 on March 12.
5.2 Situation of the earthquake disaster responses
5.2.1

Immediately after the earthquake occurred
Immediately after the earthquake, the Emergency Response Center was established at the power
station as well as Headquarters to implement countermeasures. The Emergency Disaster Response
Center at the Headquarters was company-wide and the Nuclear Power Division, as the Nuclear PowerTeam, served as a contact with the nuclear power station. The Headquarters provided support inarranging materials and equipment, food and such like, and taking procedures for recovery. The
Emergency Response Center at the Headquarters was fully occupied with trying to understand thesystem damage in the entire Tohoku District, arranging materials and equipment for recovery, dealing
with PR activities and customers; thus, the situation did not allow it to focus solely on nuclear
power-related matters.
When the earthquake occurred, the Chief Engineer of Reactors for Units 1 & 2 was in the main control
room for Units 1 & 2 to supervise a startup operation for Unit 2. In response to the plant shutdown due
to the earthquake, he instructed reactor cooling operations for Unit 1. Due to the subsequent fire at
Unit 1 and inundation at Unit 2 and such like, he gave instructions and supervision to the main control
room operators for nearly 5 hours until the measures taken were established.
In the event of the earthquake, lighting fixtures and a louver fell from the ceiling and scattered in themain control room. Moreover, part of the hanging smoke-proof barrier was damaged. Because of that,
operators took actions while wearing helmets. As handrails are installed on the control panel so that19

workers can maintain their posture during temblors, the operators held on to the handrails andmanaged to maintain their position for monitoring.
At Unit 1, the reactor automatically shut down at 14:46 due to the earthquake. A fire alarm box
activated an alarm at 14:57 and operators went to check the site. At 15:30, smoke was identified at the
turbine building basement and it was notified to the fire department at 15:41. Members of the in-housefire brigade went to the site to check the situation and extinguish the fire; however, they could not
identify where the smoke was coming from because of low visibility. Therefore they activated thecarbon dioxide extinguishment system (main oil tank room, EHC (electro hydraulic controller system),
control oil unit room, exciter room) at 17:15 after instructing operators to evacuate from the turbine
building and confirming that evacuation had been completed. As a precautionary measure, thehydrogen gas used for the generator was replaced with nitrogen gas from 16:13.
It was assumed that smoke was generated from the high-voltage power distribution panel area at the
turbine building 1st basement and operators checked the site while wearing airline masks. They
confirmed that units No. 7 & 8 for the normal metal-clad switch gear 6-1A had burnt out andoverheated among high-voltage power distribution panels and they extinguished the fire by using
seven dry-chemical extinguishers. Some roads in the vicinity of the power station were damaged dueto the earthquake and tsunami and firefighters could not enter the site. Thus workers from a
subcontractor who had experience working at the fire department confirmed fire extinguishing at 22:55.
It was assumed that short circuit and earth faults occurred within the high-voltage power distribution
panel caused by the earthquake shaking and the generated arcing heat caused the panel cable coat tomelt, generating smoke.
On March 11, an earthquake-proof new office building was under construction and the Emergency
Response Center was established within the main office building. At the Emergency Response Center,
communication equipment including a security telephone, satellite telephone, radio communicationequipment, and emergency fax were installed. In addition, the Center had an SPDS (safety parameter
display system) to monitor the plant’s parameters. Though the main office building temporarily lost
power right after the earthquake, power was secured for communication equipment and the SPDS fromCVCF (constant voltage and constant frequency) and UPS (uninterruptable power supply) and suchlike, meaning there was no particular interference to power. While the main office building lost power,
the main control room checked the plant’s parameters via telephone communication. An emergency
fax system was unavailable because NTT’s line outside the power station had been washed away. As
for the data transmission to the government via ERSS (emergency response support system), theinformation communication temporarily stopped because of a blackout at the Headquarters. A
telephone system that used a general line and mobile phone were dead and unusable. At the nearest
microwave repeater station to the power station for internal communication at Tohoku-Electric Power
Co., Inc., the emergency generator was set to be automatically activated in the event of a blackout andcommunication was maintained through replenishing fuel for the generator as needed.
It is customary to automatically declare an emergency state when an earthquake exceeding a seismic
intensity of 6 lower occurs, and individuals involved in taking countermeasures gathered at the
Response Center at the same time as the earthquake occurred in the weekday afternoon. The
Emergency Response Center consists of Response Headquarters (25 people), Information Team (7people), General Affairs Team (5 people), PR Team (3 people), Technical Team (3 people), Radiation
Control Team (4 people), Maintenance/Repair Team (10 people) and Power Generation Management
Team (3 people).
The manager of the General Affairs Division requested the subcontractor to secure personnel inaccordance with the manual for a general disaster.
The Site Superintendent notified the General Manager of the Nuclear Power Division that the reactorsof Units 1–3 had shut down. In addition, the government and local government were notified via fax.
After the earthquake, operators and division in charge of systems were supposed to perform a patrol incase of an earthquake exceeding 50 gal or a seismic intensity of 5 lower. In response to the subsequenttsunami warning, a patrol was performed to check the damage situation while cordoning off hazardous
outdoor places and securing lines of communication in case of an emergency. The main control room
members consolidated the patrol results and then notified the Emergency Response Center.
PCs, printers and such like in the main control room did not fall because they were tied down.
In the main control room, the sump water level and frequency of pump starts and stops were checked,
assuming unidentified damage to piping. Fortunately, there was no damage to the piping, so no
particular countermeasures were required.

20

1.

Secretariat

2.
3.
General Affairs Team

Emergency Disaster
Response Center
Chief: Site Superintendent

1.

Establish the Emergency Response Center andgeneral affairs
Communicate weather, damage information,
directive internally
Contact with public offices and related bodies

2.
3.
4.

Understand the damage to general affairs-related
systems
Security inside and outside the power station site
Designate and request the evacuation space
Handle other matters that do not apply to other teams

PR Team1.

Handle affairs to deal with press relations

Local General Office Team1.

Deal with local responses

Technical Team

1.

Understand damage to the Technical Division-related
systems
Handle technical matters that do not apply to other teams

2.
Radiation Control Team

1.

Understand damage to the radiation
control-related systems

Repair/Maintenance Team

1.

Understand damage to the Repair/Maintenance
Division-related systems

Electric
Maintenance/repair Team

1.

Understand damage to the Electric
Maintenance/Repair Division-related systems
Secure power
Secure communication systems

2.
3.
Mechanical Maintenance/
Repair Team
Civil Engineering TeamPower Generation
Management Team

1.

Understand damage to the Mechanical
Maintenance/Repair Division-related systems

1.

Understand damage to the Civil Engineering
Division-related systems

1.

Understand damage to the Power Generation
Management Division-related systems
Maintain security on power station facilities

2.

Figure 5.2.1-1 Emergency disaster response system and main duties

5.2.2

Immediately after tsunami arrived

A monitor TV (CRT) for an ITV monitoring panel that was placed in the main control room at Unit 2 fell
but was recovered to its original state. Footage around the cooling water intake canal was selected for the
ITV monitoring board before the earthquake; thus, the recovered monitor TV screen recorded the arrival of
the tsunami. Immediately after checking the monitor TV screen, it was found that oil from the heavy oiltank at Unit 1 had been half soaked in seawater. Then the arrival of the tsunami was promptly notified to
the Emergency Response Center. In addition, an evacuation directive was given via paging.
The tide indicator became unable to show values because of the first anaseism, and the indicator lost its
function. After the situation was resolved, it was confirmed that the time at which the tsunami reached its
maximum height was 15:29, and the height was 13 m, according to the record of the backup tide indicator.
After the tsunami arrived, NTT’s external line was disconnected and communication to the local
government via fax and telephone became unavailable. Mobile phones were also unusable. Because of that,the power station contacted the Headquarters via fax using the security line and the Headquarters provided
21

information to the government and local government.
At Unit 2, a generator excitation loss signal was sent, induced by the earthquake’s shaking and theemergency diesel generators (A), (B) and (H) automatically activated. Due to the tsunami, seawater flowed
into the reactor building closed-cooling-water system heat exchanger (B) room in the non-controlled areaof the reactor building 3rd basement, the high-pressure core spray component cooling water system heatexchanger room and stair hall to access the elevator area. And then the reactor building
closed-cooling-water system pumps (B), (D) and high-pressure core spray component cooling water
system pump were soaked. Because of that, the reactor building closed-cooling-water system pump (B)automatically stopped at 15:34 and the pertinent system pump (D), which was activated as a backup, also
stopped immediately. Thus no cooling water was supplied and the emergency diesel generator (B)automatically stopped at 15:35. Moreover, the high-pressure core spray component cooling water systempump automatically stopped at 15:41, and as a result, no cooling water was supplied to the emergency
diesel generator (H), leading to an automatic stoppage at 15:42. Seawater also invaded the reactor building
closed-cooling-water system heat exchanger (A) room.
At the spent fuel pool, a little amount of water spattered outside the pool by sloshing but the areasurrounding the indoor pool was only slightly wet. As measures taken after the Niigataken Chuetsu-oki
Earthquake, leak prevention to the outside and penetration seal reinforcement had been implemented.

5.2.3

Measures for recovery after tsunami arrived(1) Actions and operations for recovery
As power was secured and the functions of equipment required for cooling were not lost, operations forthe reactor cold shutdown were in accordance with the manual, and they led to a cold shutdown throughproper operations in the main control room.
At Unit 2, seawater that had flowed from the cooling water intake canal at the seawater pump room
entered the reactor building via an underground trench, and the seawater was discharged in two phases
by using eight temporary discharging pumps and installing a cesspool for relay. For discharging, it was
confirmed that no radioactive materials were contained by sampling the liquid beforehand. For laying
temporary cables to secure power for these discharge pumps, stored cables and materials and equipment
and those provided from the subcontractor were used. An acting Site Superintendent gave instructions
for such actions.
As large aftershocks continued, a patrol was performed as needed and the results were notified to thegovernment and local government. This patrol and communication became heavy burdens.
Directed by the Headquarters, maintaining and recovering the nuclear-related security communication
network was prioritized. Aiming for recovery, the Emergency Response Center at the Headquarters kept
inquiries to the minimum necessary level and responded to requests from the site so that the power
station could focus on recovery operations. Moreover, the minimum necessary level of TV conferenceswere held.
(2) Arrangement of personnel and materials and equipment
At Units 1 & 2, 11 operators for a training shift and 18 day shifts were present and two teams wereorganized for Units 1 & 2 and Unit 3 as supporters for main control room operations and they worked on
recovery in a two-shift system. After March 13, people who came to the site routinely and those who
could reach the site via a security line from the nearest business office gathered and assumed duties in athree-shift system.
All access roads to the power station were severed. An employee who went to Sendai for a business trip
tried to reach the power station on the day after the earthquake and used the Cobalt Line because hethought it was easy to access it from the mountain road. However, the road was impassable from Kozumi
IC and he reached the power station by walking. After that, a patrol was dispatched to check the22

condition of each access road from the power station and it found that all access roads were severed. In a
discussion between the Headquarters and Miyagi Prefecture, it was revealed that it took five days torecover the roads by using heavy equipment stored in the site for snow removal and improvement work.
Making use of the past experience to recover the severed road caused by a typhoon, the access road wasrecovered by the plant’s own initiatives.
After Day 2, people at the Emergency Response Center took breaks on a rotating basis. However, they
could not return to their homes because the roads were severed. Thus each division secured a space for asleep break, such as a meeting room within the office and training center, and they only had the clothes
they were wearing. As blankets were delivered by helicopter, they were handed to those who evacuatedfirst and site personnel received blankets from the fourth day.
Since seven days’ worth of light oil for diesel generators was in stock, no additional replenishment wasneeded. The light oil transport pipe had not been affected by the tsunami because it was installed in anunderground trench. Light oil for the emergency diesel generator (B) in Unit 2, whose function was lost
due to inundation, was used for heavy equipment. At first, extracting oil was difficult without antistatic
hoses and a 1 L bottle for sampling was used instead. In addition, light oil was used for a water carrier
and large vehicles. As there was no stock of gasoline, a few dozen liters of gasoline were transported
every day from the Headquarters by using portable cans after the land route had been recovered.
Power-supply vehicles waited in Ishinomaki because they could not reach the power station due to thedamaged access roads. Ultimately, their arrival at the power station was not necessary because Oshika
main lines 1 & 2 were recovered at 20:12 and 20:15 respectively on March 12.
The Headquarters arranged a helicopter on March 11 and used it for transportation. Originally, thehelicopter was needed to check the integrity of transmission lines after the earthquake and such like.
However, one helicopter was secured for the nuclear power station. The helicopter waited at Sendai
Airport and the manual specified evacuation to higher ground in the case of an earthquake; thus, it could
be used since it received no damage from the tsunami. In addition, securing a helicopter by contacting ahelicopter operation company, which is not a group company, was a substantial aid as a means oftransportation. The helicopter made 21 deliveries in the first three days. A heliport was located in a
distant place within the site, and packages were transported by on-site vehicles by sheer force of
numbers.
Normal mobile phones were unavailable for communication with people carrying out off-site operations,and so radio equipment was effective. In addition, PHS devices were very effective for on-sitecommunication. They were unusable in some areas because of a damaged antenna; however,communication during operation could be secured without trouble after the antennas were repaired.
Moreover, a satellite telephone was used for communication with external organizations such as the local
government (Onagawa Town). However, the number of satellite telephones at the site was small, and so
it was hard to maintain sufficient communication. The paging system receives power from an emergency
power source, and batteries when the AC power source is lost. Thus, the Manager of the Power
Generation Division directed an evacuation due to the earthquake and tsunami via the paging system.
There was no damage to radiation control materials and equipment caused by the earthquake andtsunami, and so there was no trouble with operations and no insufficiency of materials.
There were 500 people and there were three days’ worth of food and drinking water (1500 L) in stock.
However, stock for the affiliate companies and subcontractors was not sufficient. In addition, community
people evacuated to the site and only one day’s worth of food and drinking water was secured for a totalof 1,800 people as of March 12. Until recovery of the roads on March 16, food was procured from thecoverage area of Tohoku Electric Power Co., Inc. on the Sea of Japan side with less damage and thehelicopter transported food to the power station. Around one or two light meals were served a day.
The water intake and headrace systems for domestic-use water were damaged by the earthquake, and useof domestic-use water was restricted. Furthermore, the Headquarters arranged for alternate delivery

23

vehicles and obtained approval from the river manager to intake water, and the water was secured as aresult. Temporary pumps were placed in the river and the delivery vehicles transported the water backand forth and delivered approx. 400 t a day. Temporary water flowing in all lines was completed in late
April because connections of buried pipes were damaged by the earthquake and drilling the ground wasnecessary for repair.
Mainly two health promotion staff members with a nursing qualification managed the health of site
personnel and community people who evacuated to the on-site gym. A psychological counselor came tothe site to offer counseling.
Around 16:00 on March 11, local residents including the Ward Mayor who were in an isolated area dueto the severed roads requested evacuation to the PR center in the power station. However, the PR center
had also lost power and the Site Superintendent determined he would accept those people to the site. In
terms of nuclear material protection, the Ward Mayor was designated as a guarantor. Such acceptance
was notified to the government. At first, they evacuated by walking and then a bus was sent from thepower station. The number of evacuated people was 364 at the maximum on March 14. As evacuatedpeople were moving in and out, their access control was troublesome. This acceptance of community
people was notified to the local government, and the place was designated as an evacuation space of
Onagawa Town on March 16. After that, staff members of the local government were stationed there andthey dealt with community people. People who found the new evacuation place left and all left on June 6.
Until food was received from the local government, the emergency food stocked at the power station wasserved. For drinking water, the stock of plastic bottles was distributed. After recovery of the water
network within the power station, tap water was used. The Headquarters were asked to arrange daily
commodities at the request of the evacuated people and transport them by helicopter. Furthermore, thehelicopter was used to transport pregnant women and those requiring oxygen tanks among the evacuatedpeople.
Moreover, the power station accepted ships, which had sailed off the coast to avoid the tsunami, in thebay of the station because ports were damaged.
Around 02:00 on March 13, an indicative value on the monitoring post reached a peak and exceeded 5
μSv/h, which is the notification standard in Article 10 of the Special Law of Emergency Preparedness for
Nuclear Disaster. Therefore a notification was made in accordance with that. The power station was fully
occupied with recovering the plant, and so there was almost no information on the Fukushima Dai-ichi
Power Station accident. The Headquarters had a discussion with the regulatory agency to see if
notification was unnecessary because it was clear that this radiation increase was due to the impact of the
Fukushima Dai-ichi accident. However, the regulatory agency instructed the Headquarters to takeprocedures in accordance with the law and then the notification was made. After that, periodic
communication was necessary for the subsequent three months.
The Self-Defense Forces received many calls for service and contacted the power station for support
after 4 or 5 days had passed.

24

Table 5.2.3-1 Onagawa Unit 1: Plant status timeline right after the earthquake
Before the earthquake occurred: Constant-rated thermal power output operation
March 11, 2011 (Friday)
14:46

Tohoku–Pacific Ocean Earthquake occurred (seismic intensity of 6 lower observed in thesite)
Reactor automatically shut down with the seismic acceleration high (vertical direction) signal

14:47

Confirmed that all control rods were inserted
Emergency diesel generators (A), (B) automatically activated, spent fuel pool cooling &filtering system pump (A) automatically stopped

14:55

Startup transformer stopped (lockout relay operation)

14:59

Emergency diesel generators (A), (B) started loaded operations

15:00

Manually activated the reactor core isolation cooling system

15:01

Manually activated the residual heat removal system pump (A) (for suppression chambercooling operation)

15:05

Manually activated the residual heat removal system pump (C) (for suppression chambercooling operation)

15:05

Confirmed that the reactor was subcritical

15:12

Manually activated the residual heat removal system pump (B) (for suppression chambercooling operation)

15:55

Manually activated the residual heat removal system pump (D) (for suppression chambercooling operation)

16:15

Residual heat removal system pumps (A), (C) automatically stopped

AroundManually reactivated the residual heat removal system pump (A) (for suppression chamber

17:10

cooling operation)

18:29

Started reactor depressurization (by using the main steam relief valve)

AroundReactor core isolation cooling system turbine automatically stopped (due to the reactor water

19:30

level high (L-8))

20:20

Manually activated the spent fuel pool cooling & filter pump (A) (for fuel pool cooling)

21:56

Manually activated the control rod drive hydraulic system pump (A) (for supplying water tothe reactor)

23:46

Manually stopped the residual heat removal pump (A) (for preparing the reactor shutdown
cooling (flushing))
Manually activated the residual heat removal pump (A) (reactor shutdown cooling mode)

March 12, 2011 (Saturday)
00:57

Reactor coolant temperature reached 100ºC

00:58

Reactor state “Cold shutdown”

02:05

After the startup transformer received power (recovery), normal buses were recovered except
metal-clad 6-1A switch gear where fire occurred25

Table 5.2.3-2 Onagawa Unit 2: Plant status timeline right after the earthquake
Before the earthquake occurred: Undergoing the 11th periodic inspection and shifted to “startup” right before theearthquake
March 11, 2011 (Friday)
14:00

Reactor mode switch “Refueling” → “Startup” (reactor condition “Startup”)
Started control rod withdrawal

14:46

Tohoku–Pacific Ocean Earthquake occurred (Seismic intensity 6 lower observed in the site)
Reactor automatically shut down with the seismic acceleration high (horizontal direction in
the reactor building bottom) signal
Confirmed that all control rods were inserted

14:47

Emergency diesel generators (A), (B), (H) automatically activated*Due to the generator excitation loss signal
Spent fuel pool cooling & filtering system pump (B) automatically stopped
Reactor mode switch “Startup” → “Shutdown” (Reactor condition “Cold shutdown”)

14:49

Reactor building closed-cooling-water system pumps(B), (D) automatically stopped (due to

15:34

pump inundation)
Emergency diesel generator (B) automatically stopped (due to the reactor building

15:35

closed-cooling-water system (B), (D) stoppage)
High-pressure core spray component cooling water system pump automatically stopped (due

15:41

to pump inundation)
Emergency diesel generator (H) automatically stopped (due to high-pressure core spray

15:42

component cooling water system stoppage)
Manually activated the spent fuel pool cooling & filtering pump (A) (for fuel pool cooling)

20:29
March 12, 2011 (Saturday)
12:12

Manually activated the residual heat removal system pump (A) (reactor shutdown cooling
mode)

26

Table 5.2.3-3 Onagawa Unit 2: Timeline for the reactor building closed-cooling-water system B,
reactor building closed cooling seawater system B and high-pressure core spray component
cooling water system
March 11, 2011 (Friday)
14:00

Reactor startup

14:46

Earthquake occurred (seismic intensity of 6 lower observed in the site)
Reactor automatically shut down
Emergency diesel generators (A), (B), (H) automatically activated (non-loaded operation)

14:49

Major tsunami warning was issued

Around 15:21

First tsunami wave reached (visual inspection by operator)

15:34

Reactor building closed-cooling-water system pump(B) automatically stopped
Reactor building closed-cooling-water system pump(D) automatically stopped (stopped
immediately after backup pump activated)

15:35

Emergency diesel generator (B) automatically stopped with “RCW (reactor building
closed-cooling-water system) differential pressure low” signal

15:41

High-pressure core spray component cooling water system pump automatically stopped

15:42

Emergency diesel generator (H) automatically stopped with “HPCS (high-pressure corespray component cooling water system) differential pressure low” signal

Around 16:00

Operators for site inspection found inundation at RCW heat exchanger (B) room at thelowest level of the non-controlled area at the reactor building, and 3rd basement stairs (twolocations) to access the high-pressure core spray component cooling water system heatexchanger room, reactor building closed-cooling-water system heat exchanger (A) room

16:01

Manually stopped the reactor building closed cooling seawater system pump (B) (due toinundation at the reactor building closed-cooling-water system B)

16:06

Manually stopped the high-pressure core spray component cooling seawater pump (due toinundation at the high-pressure core spray component cooling water system)

Around 20:12

No radioactivity was detected in the analysis of the invaded water and found that it wasseawater

20:25

Installed temporary pumps and started discharging seawater that had flowed into the 3rdbasement of the reactor building (non-controlled area) to the outside

March 16, 2011
10:30

Completed discharging the invaded seawater27

Table 5.2.3-4 Onagawa Unit 3: Plant status timeline right after the earthquake
Before the earthquake occurred: Constant-rated thermal power output operation
March 11, 2011 (Friday)
14:46

Tohoku–Pacific Ocean Earthquake occurred (seismic intensity of 6 lower observed in thesite)
Reactor automatically shut down with the seismic acceleration high (vertical direction in
the reactor building bottom) signal

14:47

Confirmed that all control rods were inserted

14:57

Confirmed that the reactor was subcritical

15:26

Manually activated the reactor core isolation cooling system (for supplying water to thereactor)

15:28

Manually activated the reactor building closed cooling seawater system pump (D) (forsuppression chamber cooling operation)

15:30

Manually activated the reactor building closed-cooling-water system pump (B) (forsuppression chamber cooling operation)

15:30

Manually activated the residual heat removal system (B) (for suppression chamber cooling
operation)

15:43

Manually activated the reactor building closed cooling seawater system pump (C) (forsuppression chamber cooling operation)

15:44

Manually activated the residual heat removal system (A) (for suppression chamber cooling
operation)

15:45

Manually activated the reactor building closed-cooling-water system pump (A) (forsuppression chamber cooling operation)

16:40

Started reactor depressurization (by using the main steam relief valve)
Reactor core isolation cooling system turbine automatically stopped (due to the reactor
water level high (L-8))

16:57

Manually activated the reactor core isolation cooling system (for supplying water to thereactor)

21:44

Manually stopped the residual heat removal pump (A) (for preparing the reactor shutdown
cooling)

21:45

Manually stopped the reactor core isolation cooling system turbine

21:54

Water injection by the condensate supply system (for supplying water to the reactor)

23:51

Manually activated the residual heat removal pump (A) (reactor shutdown cooling mode)

March 12, 2011 (Saturday)
01:17

Reactor coolant temperature reached 100ºC
Reactor state “Cold shutdown”

28

6

Lessons learned from the earthquake disaster responses at the Onagawa Nuclear Power Station
Based on the earthquake responses at the Onagawa Nuclear Power Station, it seems there are items that may
be able to reduce operator burden at the site with some improvement, as well as items for which more
effective measures may be taken by implementing the same measures at other power stations as good practice.
These items are summarized below.
6.1 Organization, management, communication
•

•
•

•

It is necessary to establish an emergency response system that can take cross-organizational measures foreach issue at both the Headquarters and power station. Especially, preparation for logistical assistance isimportant.
Deploying a contact person between the main control room and Emergency Response Center is effective
for organizing the environment where operators can focus on plant operations.
Company-wide logistical assistance (system, local, operation transfer, life support, situational check andsuch like) should be prepared so that the power station can focus on on-site responses in the case of an
emergency.
It needs to be recognized that support from the Self-Defense Forces cannot be expected immediately
because they receive many calls for service when an emergency disaster happens.

6.2 Preparedness (system, manual, training)
•

•

•

•

•
•

•
•

•

There is a need to work on further safety improvement measures (software, hardware) including
emergency safety measures (short-term), emergency safety measures (mid and long term), and severe
accident measures.
For the case when an access road is severed, preparations should be made to handle the situation by
combining multiple transfer and supply methods by road or air (including a sea route in some situations).
In addition, it is recommended to prepare a helicopter and heavy equipment to transfer and transport
supplies and secure heavy equipment operators for the case when an access road is severed.
Even in a situation when a lighting fixture falls and scatters from the ceiling of the main control room or afire and inundation occur simultaneously at the site, training should be performed continuously for
operations in accordance with the emergency operation manual so that operators work without panicking
in order to enhance their preparation for an emergency.
Installing handrails on the control panel is very effective for helping operators to check values on
instruments and carry out operations in the event of an earthquake. Moreover, seismic countermeasures fora ceiling and supplies in a main control room, such as tying down desktop PCs and printers with belts, isimportant to manage the operation environment.
Preparation of materials and equipment such as temporary discharge pumps for building inundation caused
by a tsunami is required.
Assuming the case where a fire occurs when an access road is severed, it is necessary to arrange an on-site
extinguishment system and in-house fire brigade so that they can make an initial response in the site. The
in-house fire brigade should be trained to take actions even under conditions of low visibility caused bysmoke. Moreover, a sufficient number of oxygen tanks for a fire site needs to be prepared for a smooth
initial response.
Countermeasures should be considered for cases where contaminated water is produced within a building
in a non-controlled area.
Assuming that light oil will be used from a light oil tank of emergency diesel generators when gasoline
cannot be procured in the vicinity of the site, it is effective to prepare a diesel vehicle when renewing
vehicles at the site as well as antistatic hoses to extract the light oil.
Considering workers from subcontractors, water and food for 2,000 to 3,000 people should be secured and
29

•

•

•

so should a means of replenishing them. Water and food stocks reflect demand for affiliate companies andsubcontractors.
For outdoor and off-site operations when a mobile phone is unusable, a structure should be established toallow for communication of operation and seismic/tsunami information with the power station. It iseffective to prepare a sufficient number of radios and satellite phones.
Assuming the case where a general telephone line and mobile phone become unavailable, it is important tosecure lines of communication with the government, local governments, plant manufacturers and such like,and during outdoor operations. In order to improve the reliability of communication systems, it isrecommended to prepare a system that can receive power from an emergency power source or batteries
and develop a structure for recovery and maintenance operations in case of trouble. In addition, it isrecommended to secure a backup route for a TV conference line utilizing satellite communication. It iseffective to have different and multiple-route configurations via a microwave wireless line and opticalcommunication line (OPGW, optical ground wire) so as to secure communication reliability when using asecurity telephone and TV conference, and to preserve or maintain communication systems.
As measures to secure plant maintenance personnel, it is recommended to prepare lodgings in the vicinityof the power station and establish a night-duty system at the power station involving affiliate companies.

6.3 Initial responses in the event of the earthquake••

If access to a fire site is impossible due to low visibility, fire extinguishing using a carbon dioxide
extinguishment system is effective. Furthermore, it is necessary to make preparations in order to takecountermeasures quickly to prevent the damage from expanding immediately in the event of a fire
(replacing hydrogen gas of generators with nitrogen gas for discharge and such like).
When an earthquake occurs, it is effective to check if a tsunami is coming to the site via an ITV camera in
the main control room. It is also advisable for the office to prepare a method of checking and assessing thesituation when a tsunami is coming, induced by an earthquake.

6.4 Additional measures•••

Strengthening the reliability of off-site power is important. It is also important to enhance reliability(diversification, making other route, seismic proof and such like) of the system that supplies power froman off-site power source to the on-site emergency loads.
It is recommended to install a backup emergency power source as power for operation systems at theon-site office.
It is necessary to clarify the method of notifying the relevant parties when a tsunami is going to hit, otherthan by issuing an evacuation directive via the plant paging system, in the event of a tsunami warning. An
evacuation space such as high ground also needs to be specified, considering the possibility of alarge-scale tsunami.

30

7Overview of the Tokai No.2 Power Station
7.1 Overall layout
Tokai No.2 Power Station is located 15 km northeast of Mito City. The east side faces the Pacific Ocean
and it consists of flat ground with approx. EL 8 m. The shape of the site is almost rectangular, and the longside is the coastline. The site area is approx. 360,000 m2 including the adjacent Tokai Power Station.
A boiling water reactor is installed and it has a generator output of 1,100,000 kW.
When this disaster happened, the reactor was in constant-rated thermal power output operation.

Seawater pump areaTokai
東 Dai-ni
海 第 二Power
発電所
Station

Mountainside

SeasideMain
事務
office本館
building
東 海Power
発電所
Tokai
StationFigure 7.1-1 Overall layout of the power station

Start of operation

November 1978

Type

OutputStatus in the event of theearthquake

ReactorContainment(10,000 kW)

BWR-5Mark II

110

31

Constant-rated thermal power
output operation

7.2 System configuration
System configurations at Tokai No.2 are shown in Figure 7.2-1.
The roles of each system are as follows:•

Reactor core isolation cooling system (RCIC)
If the main condenser becomes unavailable during normal operation due to the closed main steam
isolation valve and such like, due to any cause, the system rotates the turbine-driven pumps by thereactor steam and injects the water into the reactor from the condensate storage tank to remove fueldecay heat and depressurize. In addition, the system is used as an emergency injection pump tomaintain the reactor water level in the event of a feedwater system failure and such like.

•

Residual heat removal system (RHR)
After reactor shutdown, the system cools the coolant by using pumps and heat exchangers (removalof fuel decay heat), maintains the core water by injecting cooling water in the case of an emergency(one of the emergency core cooling systems) and can bring the reactor to a cold shutdown. It has
five operation modes: the reactor shutdown cooling mode, low-pressure injection mode(emergency core cooling system), containment spray mode, suppression chamber cooling modeand fuel pool cooling mode.

•

Emergency core cooling system (ECCS)
It consists of a low-pressure core spray system (LPCS), low-pressure core injection system (LPCI),
high-pressure core spray system (HPCS) and automatic depressurization system (ADS). It removes
fuel decay heat and residual heat from the core, prevents damage to the fuel cladding caused byoverheated fuels and controls the subsequent zirconium-water reaction to a negligible level if
piping consisting of the reactor coolant pressure boundary, such as the reactor recirculation systempiping, ruptures and results in a loss of coolant accident (LOCA).

32

Reactor building
Reactor containment
Spent fuel pool
Main steam relief valve

Reactor pressure vessel

Reactor building closed-cooling-water system

Main steam isolation valve

To water discharge
canal (sea)To turbine and generator
From feedwater pump

To water discharge
canal (sea)Suppression pool

Residual heatremoval system(B)

Residual heatremoval system(A)

Low-pressure corespray system

High-pressure corespray system
Reactor core isolation coolingsystemResidual heatremoval systemseawater pump (D)Residual heatremoval systemseawater pump (B)Condensate
storage tank

Residual heatremoval systemseawater pump (A)Figure 7.2-1 Tokai No.2 System Configuration

33

Residual heatremoval systemseawater pump (C)7.3 Power source system
The electricity generated is sent to the electric power system via 275 kV transmission lines with two
circuits (Tokai Nuclear Line). This line can be used to transmit all the electricity generated at Tokai Dai-ni
and conduct the full-power operation at the power station with only one circuit even if one circuit isdamaged in an accident.
Tokai Nuclear Line with two circuits received power as the main line for reactor startup and shutdown.
Should all two circuits of this Tokai Nuclear Line lose power, the electricity for the system to safely shut
down the reactor will be supplied from the emergency diesel generator. If it takes time for recovery, a 154 kVtransmission line (nuclear line) with one circuit receives power.
Tokai Nuclear
Line (275 kV)

Nuclear line
(154 kV)

Loss of power

Loss of power

Standby transformer

Startup transformer 2 A

Tokai Nuclear
Line (275 kV)

Loss of powerStartup transformer 2B[6900 V system]

Stop[480 V system]

125 V
battery
HPCS

125 V
battery
2A

Standbycharger

125 V
battery
2B

Figure 7.3-1 Power source system, emergency power source system skeleton diagram(Condition of power source after the emergency diesel generator (2C) stoppage)

34

Power-receiving bus

8Damage caused by the earthquake and tsunami at the Tokai Dai-ni Power Station
8.1 Observation results at the Tokai No.2 Power Station
The seismic intensity was 6 lower (Tokai Mura) and the maximum acceleration observed by theseismograph at the reactor building 2nd basement was 214 gal in the south-north direction, 225 gal in theeast-west direction, and 189 gal in the vertical-horizontal direction.
It was found that the maximum acceleration observed at each floor of the reactor building fell below the
maximum response acceleration spectrum for the basic design earthquake ground motion Ss, which had
been established in accordance with the revised earthquake-proof design guideline (September 2006).
According to the response spectrum in the earthquake observation record, the record exceeded the
response spectrum for the basic design earthquake ground motion Ss locally. However, the analysis result
showed that the observed record fell under the response spectrum for the basic design earthquake groundmotion Ss in most periodic bands, including those where the natural periods for the important systems forseismic design were concentrated.
While referring to the analysis result of the reactor building based on the seismic observation record, thestructural strength and kinetic functional preservation evaluations were performed on major systems
important for seismic safety with reactor “shutdown,” “cooling” and radioactive material “containment”
functions at the Tokai No.2 Power Station in terms of impact from the seismic shaking. As a result, theobserved values in each facility found to be lower than the evaluation standard value.
The tsunami height exceeded the observation available range of the tide indicator installed inside the Tokai
No.2 Power Station Bay and data could not be acquired after around 16:40 on March 11 due to a loss of
power. Because of that, the elevation* was assumed to be approx. +4.8 – 5.3 m (EL after the crustal
deformation survey) and the runup height was assumed to be EL*+5.3 m (EL after the crustal deformation
survey) based on the trace height survey.

Observation
record NS

Control rod insertability

Reactor pressure vessel

Reactor containment

Acceleration(cm/s2)

Acceleration(cm/s2)

Acceleration(cm/s2)

Design floor
response

Residual heat removal
system piping
Main steam piping
Core support structure* The average sea surface in Tokyo Bay (T.P.) is the baseline EL.

Design floor
response

Observation
record NS

Time (s)Observation record EW

Observation
record NS

2FTime (s)Observation record EW

Reactor containment

Design floor
response

Acceleration(cm/s2)

4F

Control rod insertability

Core support structure

Reactor pressure vessel

Reactor containment

Residual heat removal
system piping
Main steam piping

Acceleration(cm/s2)

Time (s)Acceleration(cm/s2)

6F

Residual heat removal
system piping

Acceleration(cm/s2)

Observation record EWDesign floor
response

Observation
record NS

Observation record EW

2nd basement

Figure 8.1-2 Reactor building Floor response

Figure 8.1-1 Reactor building Acceleration timend

spectrum (horizontal direction, 5% decay)

history waveform (2 basement)35

8.2 Damage and impact from the earthquake
Right after the earthquake, three circuits of the off-site power source (275 kV and 154 kV) were lost.
Because of that, three emergency diesel generators (2C, 2D, HPCS) were automatically activated andsupplied power to the emergency power buses.
Vibration of the turbine bearings increased, affected by the earthquake, and the turbine generator
automatically stopped with the signal of “turbine bearings vibration large,” resulting in an automatic
reactor shutdown.
At the laboratory sump within the service building, the electromagnetic valve for the sump pump sealwater closed due to a blackout of the normal power caused by a loss of the off-site power source. Then,
seal water continued to flow into the pertinent sump and the rubber stop that blocked the pertinent funnel
was removed. As a result, flooding through the floor drain funnel occurred in the battery room of the
combination structure 1F because there was a little difference in the height of the head with the pertinent
sump. Leaked water was discharged to the emergency diesel generator room roof (outdoor) to protect thebatteries.
At the spent fuel pool, the spent fuel pool water alarm (“FUEL POOL LEVEL HI/LO”) was generated bysloshing due to the earthquake and the pool surroundings were flooded, causing the water level to decrease
by approx. 20 cm from the normal level. Therefore, the spent fuel pool was refilled by using condensate
storage tank water.
Though the water level dropped, the spent fuel rods stored in the spent fuel pool were still fully flooded(fuel top + approx. 7 m).
The spent fuel pool cooling & filtering system stopped due to a loss of the off-site power source; however,it resumed cooling with power received from the intact emergency diesel generator (2D).

8.3 Damage caused by tsunami
According to the result of checking the water trace and such like in an on-site survey, the tsunami runup
height at Tokai Dai-ni was assumed to be approx. EL*+5.3 m (EL after crustal deformation) and it did notreach major buildings because it did not exceed the site elevation at Tokai No.2 (EL*+8.0 m (EL before
the earthquake).
Though tsunami reached around the seawater pump room at the seaside of the site (EL*+3.3m (EL before
the earthquake)), inundation from above did not occur because of the side wall with EL*+6.1m (EL before
the earthquake) installed in the seawater pump rooms on the north/south sides with EL*+ Approx. 5.7m
(EL before the earthquake) as tsunami measures.
However, seawater inundation occurred in the north-side pump room from the following two points whereconstruction for waterproof had not been completed.
(1) Openings between the north-side pump room and ASW strainer area (discharge channel)(2) Non-sealed structure of cable pit*

The average sea surface in Tokyo Bay (T.P.) is the baseline (ground subsidence due to crustal
deformation was not taken into consideration).

Construction for waterproof completed at the south-side pump room.
The emergency diesel generator seawater pump (2C) stopped due to seawater that had entered thenorth-side pump room caused by the tsunami. Because of that, the emergency diesel generator (2C)
became inoperable and the emergency AC power bus 2C lost its power. Power for the emergency AC
power bus 2D and HPCS bus was continuously secured, making it possible to maintain power supply toemergency equipment.
At the inundated north-side pump room, the residual heat removal seawater pumps (A), (C) andcomponent cooling seawater pumps (A), (C) were installed in addition to the emergency diesel generator
seawater pump (2C) and these seawater pumps were flooded to the lower part of generators but there was36

no impact on their functions.
In revising disaster prevention measures in 2007, Ibaraki Prefecture made a revision to evaluate a tsunami
height to be higher than the previously estimated height. In response to that, the Civil Engineering
Division at the Headquarters proposed raising the side wall height to +6.1 m as the Japan Atomic Power
Company and started construction. Without this raising, all emergency DGs might have lost due to loss ofoff-site power and so this construction prevented a big accident.
Affected by the tsunami, all normal pieces of equipment at the sea side lost their functions, including thesea water electrolytic equipment and water intake canal dust extractor. When the tsunami hit, an earth fault
alarm for the power system was generated; however, the situation was not checked until after the next day,when the response was made, because plant stabilization was prioritized. Though only the emergency
diesel generator seawater pump (2C) lost its function among important-to-safety equipment, it caused no
problem for the plant’s cold shutdown operations.

Trace of tsunami height and range of tsunami recognitionin the Tokai Dai-ni Power Station site

Tokai Dai-niPower Station

Trace of tsunami heightobservation point
Tsunami runup range
Site boundary (including site of
Tokai Power Station)

Seawater pump area

Tokai Port
*All values are indicated in EL (EL after crustal deformation)

Figure 8.3-1 Inundation by tsunami at the Tokai No.2 Power Station
Note: This is the TP:EL value(before the earthquake)
Water intake electric room
Water intake canal dust extractor
Newly installed partition
Heightapprox. 4.0 m

Curtain wall
Heightapprox.
2.8 m

Base water level
TP 0.0m

Figure 8.3-2 Cooling water intake canal inundation by tsunami at the Tokai No.2 Power Station37

9

Responses to the earthquake disaster at the Tokai No.2 Power Station
9.1 Overview of responses right after the earthquake and tsunami for recovery and cold shutdown
The earthquake occurred at 14:48 on March 11 at the Tokai No.2 while it was in constant-rated thermal
power output operation, and the turbine generator automatically stopped because of a signal announcing“turbine bearings vibration high,” resulting in an automatic reactor shutdown. Immediately after theearthquake, all three circuits of the off-site power source were lost; however, the power to the emergency
equipment was secured as three emergency diesel generators activated. Immediately after the automatic
reactor shutdown, the reactor water level was maintained at a normal level by the reactor core isolation
cooling system and high-pressure core spray system while the reactor pressure was controlled by the main
steam relief valve. Decay heat generated after the reactor shutdown was removed through suppression poolcooling by the residual heat removal system.
After that, the emergency diesel generator seawater pump (2C) automatically stopped because of thetsunami, making the emergency diesel generator (2C) inoperable. However, the emergency power was
secured by the two other emergency diesel generators without any effect on the reactor and spent fuel poolcooling function, resulting in a reactor cold shutdown at 00:40 on March 15.
9.2 Situation of the earthquake disaster responses
9.2.1

Immediately after the earthquake occurred

As the turbine generator automatically stopped, resulting in the reactor automatic stoppage because of theearthquake, the main control room made a notification to all buildings via paging. However, most of theemployees did not understand the meaning of the notification because they were confused by theearthquake’s shaking. As the generated power indicator at the office showed zero, a power generation stop
was confirmed, and then the organization shifted to a normal trouble state. Ultimately, the organization did
not enter an emergency state or acute state, and the plant was brought to a cold shutdown under a normal
trouble state (that is, a state where the chief of the regular operation line perform operations within therange of his or her responsibility).
After receiving the first notification, the Headquarters launched the Emergency Response Center with the
President as the head and established a system to provide the necessary support, based on plant
information.
At first, the plant personnel moved to an important anti-seismic building in order to avoid being harmed bya large aftershock. However, construction of this building was planned to be completed at the end of
March and there were no pieces of communication equipment or image systems and such like. Therefore,
the plant personnel returned to the Emergency Response Center on the 3F of the main building as theconventional direction center after waiting for 30 minutes until the earthquake finished.
On March 11, the Site Superintendent was on a business trip for a meeting at the Headquarters. Because ofthat, an acting Site Superintendent took the lead in the earthquake response in accordance with the“Disaster Response Manual” until the Site Superintendent arrived at the power station via the Joban
Expressway with a patrol car as an escort.
As for coordination with the nuclear-related facilities in the Tokai area, arrangement of materials andequipment and personnel is made for an emergency based on “Tokai NOAH Arrangement.” However,
responses within the power station were sufficient without entering an emergency state.
Though three off-site power source systems of TEPCO’s substation side were lost in the event of theearthquake, three emergency diesel generators (2C, 2D, HPCS) automatically activated to supply power tothe emergency power buses. Owing to this, lighting in the main office remained lost. On the other hand,lighting, communication systems and systems to monitor the plant status were usable in the Emergency
Response Direction Center at the main building because it received power from the emergency powersource. In addition, functions of the paging system were maintained by power received from the38

emergency power source.
Communication among the Emergency Response Center, Headquarters, and main control room was madevia PHS and mobile phone. Mobile phones were effective as they were registered as a priority line in theevent of a disaster. Furthermore, major operation parameters were displayed by CRT for information
sharing at the Emergency Response Center while a TV conference system was used for information
sharing. The TV conference system was frequently shut down because of possible problems with thetelecommunications carrier. In addition, information from operators dispatched to the main control room
was written on a whiteboard for information sharing.
The Chief Engineer of Reactors made notifications to the President, and was engaged in examining
countermeasures as an assistant of the Site Superintendent at the Emergency Response Center after
checking the plant’s condition and initial response in the main control room.
In line with the water level fluctuation (lowering) after the reactor shutdown, the high-pressure core spraysystem and reactor core isolation cooling system, which are part of the emergency core cooling system,
automatically activated. Water injection function to the reactor under high-pressure was secured and so
was a sufficient reactor water level. After that, the reactor water level was maintained by the reactor coreisolation cooling system (the water source was the condensate storage tank initially and then the
suppression pool) and the reactor pressure was controlled by opening and closing the main steam relief
valve.
Since the containment isolation system activated normally, caused by the water level fluctuation (lowering)after the automatic reactor shutdown, the containment was isolated.
Similarly, the reactor building was automatically isolated by the water level fluctuation (lowering) justafter the automatic reactor shutdown. The reactor building ventilation system was switched to the standby
gas recirculation system/standby gas treatment system from the regular ventilation system in a normal way.
Procedures specified the need to dispatch the proper number of contact persons to support operators in themain control room. A few operation support group members and such like present at the office went to themain control room to offer support.
Immediately after the earthquake, many alarms were activated simultaneously in the main control room,and so each alarm was verified to see which was correct and which had been affected by the shaking.
Tying lighting louvers (hang ceilings) to each other was effective to prevent them from falling in the main
control room as earthquake measures. There were no injured persons because there were tip-resistantlockers and such like, and equipment had been tied down and fixed by anchor bolts.
Notification of the automatic reactor shutdown to the government and local government was made by fax
via simultaneous notification in accordance with the predetermined method after the earthquake andimmediate aftershock. Then, notifications to the government and local government were made as needed.
The Nuclear Safety Inspector came to the Emergency Response Center immediately after the earthquake
and checked the information as needed.
A device to control access to the radiation controlled area received power from the normal power source
and input was manually made because of a loss of functions. In addition, mobile dosimeters were
unavailable due to dead batteries, and dry-cell batteries were used until temporary power was secured. A
contamination inspection was also performed manually due to loss of a contamination monitor function.

39

9.2.2

Immediately after the tsunami arrived

While acquiring information via one-segment broadcasting on a mobile phone after the earthquake, atsunami warning was issued and people were instructed to evacuate to high ground in the entire power
station. A direction was given for subcontractors through the contact route of the General Affairs system.
Personnel were allocated to the important anti-seismic building and main building roof to serve as
watchmen. As the tsunami alarm had not been cleared, no patrol personnel were sent to secure safety until
the situation was under control, except for an operator patrol only in the buildings where the earthquake
had an intensity of 4 or higher.
Personnel present at the Emergency Response Center could not recognize that a tsunami had hit in realtime because the normal power source was used for cameras which made it possible to monitor the water
intake canal from the main control room. On the other hand, the fact that the tsunami had hit could bechecked later from a recorded image as the camera for monitoring used a different power source andreceived power.
The main control room performed monitoring with a tide indicator; however, it went down in the tsunami
after the first large wave was observed. In addition, the main control room was monitoring dischargepressure to check the seawater pump condition.
The emergency diesel generator seawater pump (2C) stopped because of seawater inundation in thenorth-side pump room, caused by the tsunami. Because of that, the emergency diesel generator (2C)
became inoperable and the emergency AC power bus 2C lost power. Power for the emergency AC powerbus 2D and high-pressure core spray system was continuously secured, maintaining power for the
emergency equipment.
Moreover, the emergency DC power supplied power to batteries via the emergency power bus and to AC
power loading. DC power, which connects to the emergency AC power bus 2C, never lost power because
the plant switched to the intact emergency AC power bus for charging batteries. Thus the DC power source
was never lost.
The residual heat removal system (B) was manually activated to remove decay heat after the automatic
reactor shutdown and it started suppression pool cooling.
While checking the flooding condition caused by sloshing of the spent fuel pool, a situational check was
performed in line with the fire alarm. However, patrol of the plant system to check for damage was
performed after the plant shutdown operations had been stabilized to some extent.
9.2.3

Plant responses after the tsunami arrived at the plant(1) Operations and actions for plant stabilization
Operations to shut down the reactor were as described in the normal manual on loss of off-site power as
power was supplied from the emergency diesel generator. Because of that, the situation did not require a
special response. Thus, operations and actions were taken under the judgment of the main control room
without any special directions from the Headquarters.
The Headquarters was especially monitoring “reactor water level, pressure, temperature,” “containmentpressure, temperature,” “suppression chamber pressure, and water level, temperature” and “environment
radiation monitor values.” The pressure at the suppression chamber just increased up to 102 kPaabs,
resulting in no emergency situation at the plant.
However, only one train of cooling system B was available and it was necessary to suspend the system inorder to switch the residual heat removal system, for suppression pool cooling, to core cooling. As coolingsystems would be lost if this switching failed, reactor cooling was initiated by switching system A toreactor shutdown cooling mode after waiting for the off-site power source and system A to be recovered
and two trains to become operable in the afternoon on March 13.
For the off-site power, 154 kV was recovered on March 13, one circuit of 275 kV on March 17 and one
circuit of 275 kV line on April 27 because TEPCO prioritized recovery of nuclear lines. Consequently, allof the off-site power was recovered. One train of the emergency diesel generator was unavailable until theoff-site power was recovered; thus, preparations were made by assuming that the operating emergency
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diesel generator may stop due to a failure and such like.
The PHS devices were of a charging type and normal power sources lost power. Therefore, PHS devices
ran out of batteries in 1 hour at the earliest, causing inconvenience in communication with the local site.
Paging and mobile phones were used.
Plant personnel accessed and checked the seawater pump area as the night came. One patroller teamconsisting of four people checked the seawater pump area with transceivers and life jackets after making
an opening in the water intake canal area fence so that they could evacuate should a tsunami hit.
Lubricating water leaked from the glands of the seawater pump and produced a pool since power for thedrain pump power installed in the pertinent area was normal system and water discharge was unavailable.
For discharging, a second team was dispatched after dawn and it discharged the water by putting anengine-driven water pump into the pool.
As for the temporary power source, the Emergency Response Center summarized the necessary powersource applied from each division and made arrangements for construction based on an order of priorities.
Half of the operators were subcontractors and half were direct-managed personnel. Any lacking cables andtools were transported from a subcontractor’s warehouses. Before construction, simple drawings andprocedures that reflected system configurations and results of an on-site check were developed.
(2) Arrangement of personnel and materials and equipment
From March 11 to the next day, all plant personnel went on duty, and after the second day a 2-shift system(12–24 hour-shift) was implemented per room.
In the main control room, two people in charge were allocated to each panel and they transferred and were
replaced with two people. The subcontractor was asked to secure the necessary personnel based on theworkload assumed by the maintenance staff. Especially, the subcontractor was asked to secure more
personnel for electrical operations.
Since the power source for the pump, which sends suppression pool water to the radioactive waste (liquid)disposal system, was a normal system, a gas turbine generator installed in the important anti-seismicbuilding was used to supply power, since the Headquarters had said this was advisable. Power-supply
vehicles were used to secure power around the main building. Some power supply vehicles were provided
by Chubu Electric Power Company, Incorporated with fuel. One week’s worth of light oil was in stock but
the Headquarters arranged for additional oil. Gasoline as a means of powering transportation could not be
procured, and the plant gave up on trying to secure it. In addition, a tanker was arranged as an emergency
source of fresh water.
Radiation control materials and equipment were sufficient, causing no trouble.
Three days’ worth of food was stocked as emergency food. After March 12, the subcontractor running theeatery delivered rice balls and the Headquarters and Tsuruga Power Station also gave support.
The Headquarters proposed goods that might be required in the site and arranged for them in response torequests from the power station.
For transporting food and materials and equipment, an application for an “emergency vehicle pass” wassubmitted to the responsible police office so that the emergency vehicle could drive along roads withtraffic regulations on a priority basis.
To carry out actions in the main building that had lost power, temporary lighting was effective.
There were no injuries requiring a special medical team, and the permanently installed health management
room was used for short sleeps and such like.

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Table 9.3.2-1 Tokai No.2: Plant status timeline right after the earthquake
Before the earthquake occurred: Constant-rated thermal power output operation
March 11, 2011 (Friday)
14:46

The earthquake occurred (seismic intensity of 6 lower in Tokaimura)

14:48

Turbine generator automatically stopped due to the turbine bearings vibration high signal andso did reactor with turbine main steam stop valve closed

14:48

Confirmed that all control rods were inserted

14:48

PCIS activated

14:48

High-pressure core spray system automatically activated

14:49

Reactor core isolation cooling system automatically activated

14:52

High-pressure core spray system injection valve automatically closed and the reactor coreisolation cooling system automatically stopped (due to the reactor water level high (L-8))

15:10

Confirmed that the reactor was subcritical

15:22

Started suppression pool cooling operation in the residual heat removal system A (manually)

15:36

Manually activated the reactor core isolation cooling system (for supplying water to thereactor)

16:42

Started suppression pool cooling operation in the residual heat removal system B(manually)

19:01

Emergency diesel generator seawater pump (2C) automatically stopped

19:21

Stopped suppression pool cooling operation in the residual heat removal system A (manually)

19:25

Manually stopped the emergency diesel generator (2C) automatically

21:52

Started reactor depressurization (used the main steam relief valve for intermittent relief)

March 12, 2011 (Saturday)
11:37

Switched the reactor water level control from the reactor core isolation cooling system to thehigh-pressure core spray system

13:11

Manually stopped the reactor core isolation cooling system (due to lowering reactor pressure)

March 13, 2011 (Sunday)
19:37

On-site power source started receiving power from the off-site backup power source (154 kV)March 14, 2011 (Monday)
23:43

Residual heat removal system A started operation in the reactor shutdown cooing mode

March 15, 2011 (Tuesday)
00:40

Reactor coolant temperature reached 100ºC (Reactor state “Cold shutdown”)

March 17, 2011 (Thursday)
15:20

Charged 275kV Tokai Nuclear Line 1

March 22, 2011 (Tuesday)
22:10

Emergency diesel generator (2C) standby (completed seawater pump 2C inspection)

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April 27, 2011 (Wednesday)
16:29

Charged 275 kV Tokai Nuclear Line 2

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10 Lessons learned from earthquake disaster responses at the Tokai No.2 Nuclear Power Station
Based on the earthquake response at Tokai No.2, it seems there are items that may be able to reduce operator
burden at the site with some improvement, as well as items for which more effective measures may be taken
by implementing the same measures at other power stations as good practice. These items are summarized
below.
10.1 Organization, management, communication
•

•

•

•

It is necessary to predetermine the communication system with subcontractors for an emergency andestablish a response system once an emergency happens. Utilizing this system, information should beshared sufficiently.
The Site Superintendent was absent when the earthquake and tsunami occurred and an acting Site
Superintendent took the lead in handling the accident. The acting Site Superintendent could properly
handle the situation because the emergency response exercises were conducted on a daily basis withparticipation of all the management members.
An internal system and way of coordinating with the mass media and local government should be arranged
beforehand to distribute information to the local area and multiple methods should be prepared for
notification.
For sharing information between the Emergency Response Center and main control room, the Emergency
Response Center should be able to monitor the process computer, local monitoring camera (ITV) and tidegauge indications. Using a TV conference system is also effective. In addition, a method to acquire
information from external bodies (TV and radio) should be prepared.

10.2 Preparedness (system, manual, training)
•

•
•

•

•

Plant workers who are not required for the accident response should be evacuated. An evacuation route,method and place shall be predetermined and fully notified to workers on a routine basis. When evacuatingfrom the controlled area, evacuation should be prioritized and a contamination inspection should be
performed at the evacuation area. Paging is effective as a method to notify people of a situation that
requires evacuation and a beeping sound should be clear so as to notify people of an emergency.
Furthermore, preparations should be needed through exercises as needed to ensure evacuation. In addition,
preparations should be made for emergency evacuation guidance for visitors at the power station facility.
The management should secure lines of communication with subordinates for an emergency. E-mail iseffective and a way to confirm safety should be determined beforehand.
An immediate installation of a temporary power source can be possible by organizing a number of backup
cables, specifications, storage locations, clarifying the storage location of related materials and equipment,
amount and pre-check method, developing lists of equipment power source, MCC unit list (specificationsof built-in equipment such as NFB) and storing the latest paper-based drawings (CWD and such like) near
the local site. Moreover, it is effective to deploy mobile engine generators in groups of different voltages
and capacities.
Three days’ worth of water and food were stocked for disaster response personnel for an emergency.
However, more personnel were engaged than expected as the earthquake and tsunami hit in the evening on
a weekday. Moreover, those who could not return to their home stayed at the power station. Thus, there
was insufficient water and food. Because of that, supplies were transported from the Headquarters (Tokyo)
and Tsuruga for replenishment. Considering operators from subcontractors, water and food for 2,000 to
3,000 people should be secured and a replenishment method should be prepared beforehand.
As the emergency diesel generator and power source vehicles consume a large amount of fuel during theircontinuous operation, their usage should be determined beforehand and a fuel stocking and replenishment
method should be prepared in line with their usage. It is necessary to stock fuel in a location that will not
be affected by a tsunami.

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•

•

It is recommended to make a list of supplies that are required if the emergency situation is prolonged and to
replenish supplies immediately through information sharing with the Headquarters. The same applies tologistical matters such as temporary lavatories.
It is recommended to take full tip-resistant and anti-drop measures for supplies in the event of anearthquake.

10.3 Initial responses in the event of an earthquake
•
•
•
•

•

A team should be organized to dispatch operators to the site for duties such as patrol and it is necessary to
pay attention so that operator does not act alone in terms of safety.
Each personnel member should prepare supplies required for the emergency, such as helmets andflashlights as much as possible.
Developing procedures and sharing information beforehand is essential for smooth and reliable operations
even in a situation requiring urgent action.
For contact to the main control room from the site, information management for the specific contact person
makes it possible to avoid conflicting information. It is recommended to ensure this through exercises
beforehand.
When working in an area where operators cannot hear a pager or check the sea condition, such as theseawater pump room, allocating an exclusive observer is recommended to secure the safety of operators,
not just by preparing a mobile phone constantly.

10.4 Additional measures
•

•
•

•

•
•

Through a follow-up of a revision to the disaster prevention plan carried out by the local government,
raising the side wall in the seawater pump area resulted in securing power and settling the situation without
it escalating into an accident. It is an example that shows the importance of taking safety measures in atimely manner without hesitation.
During recovery operation after the earthquake, thorough information sharing among the parties concernedis required in the morning and at meetings for smooth operations.
It is recommended to make preparations to receive power from emergency power sources and batteries so
as to enhance the reliability of communication systems. In addition, diversification of communication lines
such as by using a satellite line for a TV conference system is desirable. Lines of communication should
also be diversified by utilizing wired land phones, charging-type simple telephone devices, radios andtransceivers and such like. Moreover, mobile phones should be registered for the disaster priority line.
Information sharing is easy with a telephone with a speakerphone function.
It is recommended to make arrangement so that a radiation control calculator, weather observation device,and local monitoring camera necessary for disaster measures can receive power from an emergency powersource with redundancy as much as possible.
Preparations should be made to develop a list of instruments required for monitoring in an emergency inthe main control room and to connect power sources such as batteries immediately.
Reuse of PHS antennas is impossible if they are inundated; therefore, reserving backup antennas isrequired. Moreover, the locations of antennas should be decided in light of the impact of an earthquake andtsunami.

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11 Conclusion
Currently, 49 nuclear power plants in Japan have been shut down except Ooi Units 3 & 4. In July 2013, theNuclear Regulation Authority promulgated new technical standards and a plant re-startup is now possible
only after confirming whether important-to-safety measures are appropriately taken.
However, such efforts of the Nuclear Regulation Authority only are not enough to overcome the public’s
distrust in nuclear safety and it may be necessary for electric companies to show their attitude to address
safety enhancement over the long term. In conjunction with proficiency in accident response through training,
it is necessary to combine measures in hardware (e.g., system expansion to prevent accident spreading) andsoftware (e.g., developing a response manual in accordance with the accident condition) and continuously
promote activities to cultivate a safety culture that does not degrade as time passes.
The Japan Nuclear Safety Institute hopes this report will serves as an aid in the activities of electric powercompanies.

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